Question

Find the power of a convex lens which forms a real and inverted image of magnification $-1$ of an object which is placed at a distance of $20cm$ from its optical centre.

Answer 1

Given magnification $m=-1$ and object distance $u=-20cm$

We know magnification $m=\frac{-v}{u}$

$v=-mu=(-1)(-20)=20cm$

Now from lens formula we have,

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{20}-\frac{-1}{20}=\frac{1}{f}$

$f=10cm$

Now power$p=\frac{1}{f}=\frac{1}{\frac{10}{100}}=10$ D